Photolithography is a process used in microfabrication to pattern the bulk of a substrate. It uses light to transfer a geometric pattern from an optical mask to a light-sensitive chemical “photoresist,” or simply “resist,” on the substrate. The pattern in the resist is created by exposing it to light with a projected image using an optical mask.
Optical proximity correction (OPC) is a photolithography enhancement technique commonly used to compensate for image errors due to diffraction or process effects. OPC corrects image errors by moving edges or adding extra polygons to the pattern written on the optical mask. Model based OPC uses compact models to dynamically simulate the final pattern and thereby derive the movement of edges, typically broken into sections, to find the best solution. The objective is to reproduce, as well as possible, the original layout drawn by the designer in the silicon wafer.
The cost of manufacturing advanced mask sets is steadily increasing as technology becomes more and more complex. In addition, turn-around time is always an important consideration in semiconductor manufacturing. As a result, computer simulations of the photolithography process, which assist in reducing both the cost and turn-around time, have become an integral part of semiconductor manufacturing.
As semiconductor feature sizes continue to shrink further below the exposure wavelength, mask topography effect, also called thick mask effect or mask three-dimensionality (3D), becomes an increasingly significant factor impacting the photolithography modeling and full chip OPC process. Off-axis illumination (OAI) is an optical system setup in which the incoming light strikes the optical mask at an oblique angle rather than perpendicularly. That is to say, the incident light is not parallel to the axis of the optical system. OAI brings additional complication to the photolithography simulation of mask topography effect.
One of the most important inputs to any photolithography simulation system is the model for the interaction between the illuminating electric field and the mask. Among different types of modeling schemes for tackling OAI, Abbe's method is usually considered to be accurate. Abbe's method finds the light intensity at the resist for each single point on the light source and then integrates them together. Therefore, Abbe's method runs extremely slowly, making it impractical for full chip level implementation. Instead, application of Abbe's method is limited to small areas of a chip design layout.
In Hopkins' method, the integration over the light source is done before calculating the inverse Fourier integrals. The major advantage of Hopkins' method is that source-dependent information is completely independent of the mask function and can be pre-computed and stored, thus reducing much of the computational effort during lithography simulation. However, because source dependent information and mask geometry are mixed together in the situation of OAI and multi-tone masks, Hopkins' method can be very inaccurate in light intensity calculation.
The prior art of the filter-based mask 3D modeling schemes uses scaling parameters to generate equivalent mask field. Filter-based models with fitting parameters have obvious shortages. For example, the calibrated parameters are pattern dependent, transmission and phase are fixed (which should be angle-dependent), the fitting parameters for edge-coupling and OAI parameter can hardly co-exist, and multi-tone mask might not be calibrated well because different tones have different behavior. Consequently, conventional filter-based modeling schemes cannot accurately perform photolithography simulation to support OAI and multi-tone masks.
A hybrid approach splits the light source into several regions, and processes each region individually (e.g., by using filter-based models). The hybrid approach is a compromise between Abbe's method and Hopkins' method. It is faster than Abbe's method, but not as accurate as Abbe's method. The hybrid approach is also several times slower than the filter-based modeling schemes.